This invention relates to a process for the grafting on to brominated butyl rubbers of polymers based on conjugated diolefin monomers and the use of these graft copolymers in rubber compositions that, upon vulcanization, exhibit improved physical properties. The graft copolymers are particularly, but not exclusively, suited for use in tire tread compositions.
With the increasing demand for automobile safety and low fuel comsumption, specifications for rubber compounds for tire treads have become more demanding. Tire treads are required to be very tough and very wear resistant, to have a high degree of traction on both wet and dry surfaces, to provide low rolling resistance and heat build up and to retain their rubbery characteristics over a wide temperature range. However, some of these requirements are essentially incompatible with other requirements.
The addition of a butyl rubber to the tread formulation of a tire leads to an improvement in the wet skid resistance of the tire tread but there is a concomitant reduction in the wear resistance of the tire tread. Thus it would be desirable if the butyl rubber could be modified in a manner such that, when used in a tire tread formulation, the improved wet skid resistance is retained and, additionally, there is improved wear resistance while a desirable balance of the other physical properties is maintained.
As butyl rubbers have a very low level of unsaturation they do not have good compatibility with highly unsaturated rubbers such as polybutadiene or styrene-butadiene copolymers. Consequently several different grafting procedures have been developed by means of which further unsaturation may be introduced.
U.S. Pat. No. 5,264,494 discloses one process for preparing graft copolymers of halogenated butyl rubbers and polymers based on conjugated diolefin monomers. This process involves use of a solution of a chlorinated butyl rubber or a brominated butyl rubber, in a inert solvent, and a solution of a living, alkali metal terminated polymer based on conjugated diolefin monomers. This process is expensive and disadvantageous, as the living polymer is extremely sensitive to moisture and to impurities, so extreme precautions must be taken to protect it, the reaction mixture containing it and the halogenated butyl rubber, from moisture and impurities. Hence, the inert organic solvent or solvents must be moisture-free, and the reaction must be carried out under an inert atmosphere, for instance under nitrogen. Furthermore, ensuring that the polymers to be grafted are not contaminated with impurities can involve several washing steps, and the solvent or solvents must be removed from these washing steps and from the product after the reaction. For environmental reasons, it is desirable to avoid use of large volumes of organic solvents where possible.
U.S. Pat. No. 5,342,896 also relates to the preparation to graft copolymers composed of a halobutyl rubber and a polymer based on a diene monomer. This process uses a lithium-terminated diene-capped vinyl aromatic polymer that is reacted with the halobutyl rubber. The lithium-terminated diene-capped vinyl aromatic polymer can be prepared by adding a small amount of diene monomer to a solution of living lithium-terminated polystyrene. This reaction is carried out in an inert organic solvent such as cyclohexane and under an inert atmosphere. Again, precautions against moisture and impurities must be taken, in view of the moisture sensitivity and sensitivity to impurities of the living polymer and, again, organic solvents are used.
The present invention provides a process for preparing graft copolymers of brominated butyl rubbers and polymers based on conjugated diolefin monomers that does not involve use of living polymers and, therefore, avoids the disadvantages that accompany their use. It also avoids use of organic solvent in the formation of the graft copolymer.
The present invention provides a process for preparing a graft copolymer of a brominated butyl rubber and a polymer based on a conjugated diolefin monomer, which process comprises mixing a solid brominated butyl rubber with a solid polymer which is based on a conjugated diolefin monomer and which also includes some Cxe2x80x94Sxe2x80x94(S)nxe2x80x94C bonds, where n is an integer from 1 to 7, the mixing being carried out at a temperature greater than 50xc2x0 C., for a period of time sufficient to cause grafting.